The present invention relates to reducing electromagnetic interference (EMI) generated by computer systems generally, and more particularly to reducing EMI by spreading the emission spectrum of pixel clock signals and pixel information provided to CRTs and digital displays.
Electronic equipment sold in the United States typically must meet certain regulations regarding the emission of electromagnetic interference. These and other regulations are generated by the Federal Communication Commission. They help equipment to be used in proximity with other equipment without impairing the other equipment's operation. Other regulatory bodies, such as the European Union have similar requirements. The requirements of these regulations are beyond the scope of this document.
If these emission standards are not met by an initial design of an electronic equipment product, several costs are incurred. For example, the product may be delayed, resulting in lost opportunity costs. Also, corrective action may need to be taken, such as the incorporation of more complicated and expensive components and shielding. Further testing is likely to be required to ensure compliance, with its associated costs and delays.
EMI is caused by signals being transferred around electronic systems, particularly from one electronic component in the system to another. Generally, the stronger a signal component at a particular frequency, the higher the EMI at that frequency. The peak EMI spectral component is typically of most concern and the component that needs to be reduced to achieve compliance.
One type of electronic equipment that receives much attention from an EMI point of view are monitors such as cathode-ray tube monitors (CRTs) and digital displays, such as flat panel monitors. In particular, the signals that drive these monitors, video signals, create EMI that may cause compliance difficulties.
Conventional solutions to this problem include the use of filters or chokes that reduce the edge rate of the video signal. Adding shielding may make further improvements. But these add costs and can degrade image quality.
Thus, what is needed are circuits, methods, and apparatus that provide a reduction in the maximum EMI generated by video signals without adding costly devices or shielding, and without degrading image quality noticeably.